We have discussed how the tetrahedral structure and dynamical properties of water are varied with pressure and temperature, focusing on the hydrogen-bonding interactions and the molecular packing efficiency. Anomalous properties of water are controlled by the strong and directional hydrogen bonds which are bent by an increase in pressure or temperature. It is shown that the hydrogen bonds are not completely broken in hot dense water near the critical temperature.
The microscopic structure of sub-and supercritical water has recently been determined by novel liquid X-ray diffraction using an imaging plate detector and neutron diffraction with isotopic substitution. The X-ray and neutron results are compared with those obtained by Monte Carlo and molecular dynamics computer simulations and Raman and infrared spectroscopy. A recent attempt to obtain the three-dimensional structure of supercritical water by spherical harmonic expansion method is also described Discussion is focused on the degree of hydrogen bonding in supercritical water and its correlation with physicochemical properties.
Recent high-pressure and low-temperature experiments have properly demonstrated polyamorphism and the first-order transition between the low-and high-density amorphous ices. Experimental and theoretical results further suggested the exsistence of a phase transition, as well as a critical point, in supercooled liquid water. The phase behavior of these amorphous ices and its relation to the liquid water are discussed.
The pressure dependences of sound velocities, refractive index and elastic constants in ice-VII phase have been determined up to 8 GPa at 300 K by high-pressure Brillouin spectroscopy developed for a single molecular crystal grown in a diamond anvil cell. The Cauchy relation of C12=C44 was found at each pressure. This result suggests that the interactions between the oxygen atoms can be described by central forces, which plays one of key roles in the extreme pressure stability in the ice-VII phase.
Two resonance phenomena have been observed in ice-VII phase by infrared absorption measurement to 110 GPa: Fermi resonance between the stretching and the overtone of the bending vibration around 25 GPa and Fano unti-resonance related to the anharmonic stretching vibration above 40 GPa. These spectral features provide new insight into the pressure variation and the symmetrization mechanism of the hydrogen bond.
Pressure studies of phospholipid bilayer membranes are reviewed. The effect of pressure on the phase transition temperatures of dipalmitoylphosphatidylcholine bilayer membranes were determined by the various physical techniques including dilatometry, ESR, X-ray diffraction, fluorescence, Raman spectroscopy, neutron diffraction, light transmission, and NMR. In addition to the liquid crystal, ripple gel and lamellar gel phases, a new pressureinduced, interdigitated gel phase is presented.
Recent discovery of C60 polymers is reviewed. Application of high pressure (5 GPa) and high temperature (∼ 800°C) to pure C60 produces a new solid form with a rhombohedral unit cell. In this crystal, neighboring C60 molecules are bonded by four membered rings forming a two-dimensional polymer of C60 Different conditions of pressure and temperature produce other polymeric forms. Alternative approaches for synthesizing C60 polymers are compared with the high pressure synthesis.
Simple high pressure apparatus has been designed to measure physical properties of condensed matters at high magnetic field. The magnetoresistance of borocarbide is briefly reported as an example of measurement. It is emphasized that this type of measurement is very important for understanding the electronic structureof metals and alloys.
The transient grating method with a high pressure optical cell is presented for determinations of sound velocity, sound absorption coefficient and thermal diffusion constant of supercritical fluids. By choosing proper probe molecules which are photochemically active, mass diffusion constants of transient molecules in supercritical fluids are also measured.